Coated magnetic recording media are extensively used for a wide variety of applications, including audio recording, video recording, data storage, and the like. Coated magnetic recording media typically comprise at least one magnetizable layer coated onto a nonmagnetizable support. The least expensive and most widely used magnetic recording media are particulate media, in which the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder.
Recent years have seen a marked increase in the density of magnetic recording. To function effectively at higher recording densities, magnetic pigments must have small particle size (smaller than the smallest "bit" of magnetic information), high coercivity (resistance to demagnetization), and high magnetization (magnetic moment per unit volume). Because magnetic recording media may be used to store irreplaceable information, magnetic pigments must also have exceptional stability when exposed to repeated mechanical stresses and to changes in environmental conditions, e.g., changes in temperature, humidity, and pollutant levels.
Hexagonal ferrite particles with uniaxial anisotropy have gained attention as a candidate pigment for high-density magnetic recording. These particles are typically platelet-shaped with diameter:thickness ratios in the range from 3:1 to 15:1. Because of their strong uniaxial magnetocrystalline anisotropy, which is perpendicular to the plane of the particle, even very small hexagonal particles, less than 0.05 micron in diameter, show relatively high coercivity values. Moreover, because hexagonal ferrite particles are comprised of metal ions which are, typically, all in the fully oxidized state, the particles are very stable chemically. For perpendicular recording, hexagonal magnetic ferrite particles may be aligned such that the platelets are oriented parallel to the substrate. Alternatively, excellent electromagnetic performance may also be achieved with longitudinally oriented and unoriented particles.
For recording purposes, single domain, hexagonal ferrite particles of the magnetoplumbite or M-type structure have been the most widely used type of hexagonal ferrite. In unsubstituted form, M-type hexagonal ferrite particles have the chemical formula B(II)Fe.sub.12 O.sub.19, where B(II) may be Ba, Sr, Pb, or (in part) Ca, and the iron is all in the trivalent or ferric state (Fe(III)). In Smit and Wijn, Ferrites, Wiley, New York, 1959, pp. 180-184, the magnetoplumbite structure is described as a --S--R--S--R--S--R-- construction, where S corresponds to a block containing only Fe and O ions in a spinel structure, R is a block containing the B(II) ion, and the ratio of S blocks to R blocks is 1:1. Another hexagonal ferrite structure known to have uniaxial magnetic anisotropy is the W--structure, which can be represented as an --S--S--R--S--S--R--S--S--R-- construction, for which the ratio of S blocks to R blocks is 2:1.
Unsubstituted, single domain M-type hexagonal ferrite particles have a coercivity of 4000 to 6000 .phi.e, which is too high for most magnetic recording applications. To lower the coercivity of the M-type ferrites to a value more suitable for magnetic recording applications, e.g., 300 to 3000 .phi.e, 2, 4, 5, or 6-valent metal ions have been substituted for a portion of the Fe(III) in amounts such that the average valence of the substituted ions is 3 and such that the molar ratio of the Fe(III) and substituted ions to B(II) ions is 12:1. See O. Kubo et al., "A Study of Substitution Elements for Barium Ferrite Particles for Perpendicular Magnetic Recording," J of the Magnetics Society of Japan, v. 12, Suppl. S1 (1989), p. 875. This approach shall be referred to herein as the "linked substitution" method.
For example, U.S. Pat. No. 4,341,648 describes compositions such as B(II)M(II).sub.x M(IV).sub.x Fe.sub.12-2x O.sub.19 and B(II)M(II).sub.2y M(V).sub.y Fe.sub.12-3y O.sub.19, where M(II) is Co, M(IV) is Ti and Ge, and M(V) is V, Nb, Sb, and Ta. Similarly, U.S. Pat. No. 4,543,198 describes the composition B(II)M(II).sub.x M(IV).sub.x Fe.sub.12-2x O.sub.19, where M(II) is Co, Ni, and Zn, and M(IV) is Ti, Zr, and Hf. U.S. Pat. Nos. 4,529,542, 4,664,831, 4,778,734, and 4,810,402 describe similar compositions.
A drawback of using the linked substitution method for reducing the coercivity of M-type hexagonal ferrites is that the substitution of the nonmagnetic M(IV) or M(V) ions for the magnetic Fe(III) ion decreases the magnetic moment of the resulting particles. This effect becomes more dramatic as the size of the particles is decreased inasmuch as increasing the degree of substitution increases the rate at which the magnetic moment decreases with decreasing particle size. See, for example, O. Kubo et al., J. Appl. Phys., v. 57 (1985), p. 4280; and U. Meisen and A. Eiling, IEEE Trans. Mag., v. 26 (1990), p. 21.
One approach that has been proposed for increasing the magnetization of M-type hexagonal magnetic ferrite particles is to deposit a layer of a magnetic ferrite spinel, such as magnetite, cobalt, zinc, or nickel ferrite, or solid solutions thereof, on the surface of M-type hexagonal ferrite particles. This can be done either directly, as in U.S. Pat. No. 4,584,242, or by an indirect method, as in U.S. Pat. Nos. 4,778,734, 4,806,429, and 4,851,292. In the indirect method, after M-type hexagonal ferrite particles are formed, they are reacted with an aqueous solution containing a divalent element, such as Zn, and then annealed at high temperature to form a spinel-like outer layer which is deficient in Ba. The disadvantage of the indirect method is that it requires a complicated and expensive series of processing steps in which high-coercivity M-type hexagonal ferrite particles must first be produced and then reacted and annealed to form the sandwich-type particles.
Other magnetoplumbite/spinel composites have been described. For example, U.S. Pat. No. 4,855,205 describes a sintered, two-phase composite containing a ferrite spinel phase and a magnetoplumbite phase. These composites have the composition B(II).sub.1-x R.sub.x Fe.sub.12 O.sub.19, where R is a trivalent rare earth element. The two-phase composite has a lower magnetic moment than that of single phase magnetoplumbite particles and is used as a carrier for electrophotographic processes. The lower magnetic moment is desirable for electrophotography, but is undesirable for magnetic recording.
In another approach, hexagonal magnetic ferrites having a modified magnetoplumbite structure have been proposed. The modified magnetoplumbite structure is a structure having a ratio of S blocks to R blocks of greater than 1, but less than 2. The modified magnetoplumbite structure thus is intermediate in structure between the M-type and W-type structures.
For example, U.S. Pat. No. 4,886,714 describes hexagonal magnetic ferrite particles having a modified magnetoplumbite structure. To prepare such particles according to U.S. Pat. No. 4,886,714, a precursor powder is first formed by a coprecipitation method. The precursor powder is then fired at 650.degree. C. to 850.degree. C. in a non-oxidizing atmosphere, i.e., either in an inert atmosphere containing an inert gas such as nitrogen or argon and no oxygen, or in the presence of a reducing agent such as carbon powder or hydrogen. According to U.S. Pat. No. 4,886,714, only hexagonal ferrite of the stoichiometric composition (i.e., the substituted or unsubstituted M-type structure having an S block to R block ratio equal to 1) will be obtained if firing occurs in air, and any excess Fe ions added with the intention of forming the modified magnetoplumbite structure will instead be converted into the nonmagnetic alpha ferric oxide, or hematite, phase. When this happens, the magnetization of the resulting particles is reduced. U.S. Pat. No. 4,886,714 adds that, if firing is carried out in air, firing must occur at a temperature over 850.degree. C.